The present invention relates to a gear transmission mechanism, and more particularly relates to a gear transmission mechanism which can be used in an automatic transmission for an automotive vehicle such as a passenger car.
Many automotive vehicles in these days are fitted with automatic transmissions. Such an automatic transmission typically includes a fluid torque converter and a gear transmission mechanism, the torque converter providing supply of rotational power to the gear transmission mechanism. The gear transmission mechanism, in turn, alters the speed ratio of the rotational power which it receives, and supplies said rotational power to a final drive device such as a differential device. Such gear transmission mechanism is capable of providing a plurality of different speed stages or speed ratios between its rotational power input member and its rotational power output member, and is controlled to provide one or another of these speed stages by a transmission control device, which receives information relating to the operational conditions of the vehicle incorporating the transmission, such as information on vehicle throttle opening and vehicle road speed for example, and which makes a decision as to what speed stage should be provided by the gear transmission mechanism, at each particular time.
Such a gear transmission mechanism, therefore, is required to provide a plurality of speed stages, which naturally should be of proper and appropriate ratios. Further, the gear transmission mechanism is required to be mounted in the vehicle, and to be compact. Particularly in the case that the vehicle is one in which the engine and transmission are mounted together in the front thereof transversely to the vehicle, which is a typical front wheel drive configuration or a so called FF configuration, the requirements for compactness of the gear transmission mechanism become very severe. Particularly, severe requirements are made for the gear transmission mechanism to be short in its axial length.
Further, as a matter of course such a gear transmission mechanism should provide a good driving feeling when shifting between the speed stages, without undue shift shock or over revving of the engine of the vehicle during shifting. The vehicle should have good drivability. Further, it is important that the durability of the gear transmission mechanism should be good, especially the durability of the friction engaging mechanisms thereof.
A gear transmission mechanism is typically usually used in one high speed stage much more than in the lower speed stages; this may be called the top speed stage. It is very important that the durability of the gear transmission mechanism in this so called top speed stage should be particularly good, since the gear transmission mechanism is kept in said top speed stage for such a large proportion of the time. Further, often a gear transmission mechanism in a modern vehicle is provided with a so called overdrive speed stage, which is a higher speed stage than the above defined top speed stage. Such an overdrive speed stage is used during expressway driving or the like. Since such an overdrive speed stage is also quite frequently used for long periods, it is also important that the durability of the gear transmission mechanism in this so called overdrive speed stage should be quite good.